Browsing C++ and coming up mostly with nodejs libraries. http://libraries.io/search?languages=C%2B%2B&order=desc&sort...
The closest thing to rubygems might be Portage (1) or AUR (2). You might be able to search each of those for "c++". Or maybe peek into the constituent files for '.c' or '.cpp', '.cxx', etc.
1) http://en.wikipedia.org/wiki/Portage_%28software%29
2) https://wiki.archlinux.org/index.php/Arch_User_Repository